Opto-mechanical device

ABSTRACT

An opto-mechanical device is constituted by an optically responsive actuating member in which a mechanical strain occurs in response to changes in its illumination. The actuating member is preferably at least partly formed from a semiconducting glassy matrix such as a chalcogenide glass. Preferably the opto-mechanical device illuminates the actuating member with light which is preferably plane polarized. Preferably it also varies the polarization angle of the plane polarized light to cause a change in the strain generated in the actuating member.

BACKGROUND TO THE INVENTION

With piezo-electric materials when an electrical field is applied acrossthem a mechanical strain results. This mechanical strain results inexpansion or contraction of the piezo-electric material. Accordinglypiezo-electric materials can be used to provide mechanical movement inresponse to an applied electric field. They are particularly used whereonly small movements are required, for example, on the stages ofscanning electron microscopes or in micro-manipulators.

STATEMENT OF THE INVENTION

According to this invention an opto-mechanical device comprises anoptically responsive actuating member in which a mechanical strainoccurs in response to changes in its illumination.

The optically responsive actuating member is preferably at least partlyformed from a semiconducting glassy matrix. Preferably thesemiconducting glassy matrix includes a group VI element alone or incombination with at least one element from the same or a different groupof the periodic table. Such material is often referred to as achalcogenide glass. Whilst the elements forming the semiconductingglassy matrix may be in stoichiometric proportions it is preferred thatthey are in non-stoichiometric proportions. Other materials such asphoto-active polymers which demonstrate photo-anisotropy can also beused.

Preferably the opto-mechanical device includes means to illuminate theactuating member. The illumination means may be formed by an opticalwaveguide or may include a light emitting source arranged to illuminatethe actuating member directly or via a coupling fibre or lens, or maysimply be a window in an otherwise opaque housing or package containingthe device to allow light to impinge upon the actuating member. When theillumination means includes an optical waveguide the illuminating lightmay be evanescently coupled to the actuating member or the output fromthe waveguide used to illuminate the actuating member. It is alsopossible for the actuating member itself to act as a waveguide and, inthis case, the illumination means may be coupled to the actuatingmember.

The mechanical strain in the actuating member resulting from itsillumination is provided by two different effects. Firstly there is ascalar effect in which when an initially isotropic semiconducting glassymatrix is illuminated by unpolarised light it expands. This expansionproduces a metastable state and the semiconducting glassy matrix can bereturned to its isotropic state by heating. An opto-mechanical deviceusing this scalar effect has only limited application.

The inventors of the present application have discovered that there isalso an additional vectorial change in the mechanical strain generatedin the actuating member when it is illuminated with polarised light.When the semiconducting glassy matrix is illuminated with polarisedlight having its electric vector in a predetermined direction thisproduces a strain in the matrix which causes it to contract in thepredetermined direction. Conversely, illuminating the semiconductingglassy matrix with light polarised to have its electric vectororthogonal to the predetermined direction produces a strain in it whichcauses it to expand in the predetermined direction. This expansion orcontraction is again a metastable state of the actuating member and thismetastable state can once again be relieved by the application of heat.However, much more usefully the metastable state can be changed bymerely changing the orientation of the polarisation of light being usedto illuminate the actuating member. Accordingly, by changing the stateor angle of polarisation of the illuminating light a correspondingchange in the mechanical strain of the actuating member occurs and hencea corresponding change in its contraction or expansion can be made tooccur. This change is completely reversible and repeatable and there is,for example, no hysteresis as occurs in piezo-electric actuatingmembers.

Preferably therefore the opto-mechanical device also includes means toilluminate the actuating member with a source of polarised light andpreferably includes means to vary the angle of polarisation of the lightapplied to the actuating member. In this case the means to illuminatethe actuating member is preferably polarisation-preserving and mayinclude a polarisation-preserving optical fibre and may include anelectro-optical modulator to vary the angle of polarisation of the lightapplied to the actuating member. Other ways of changing the angle ofpolarisation of the light applied to the actuating member includeswitching between light of perpendicular polarisation states byswitching between differently polarised light sources or by, forexample, switching a half wavelength plate into the optical path.

The linear expansion or contraction of the semiconducting glass matrixon its illumination is only small, for example of the order of1:200,000. The change also only occurs in regions which are illuminatedby the light and so tends to occur only in a reasonably shallow surfacelayer of the glassy matrix into which the illuminating light penetrates.It is believed that, in the case of chalcogenide glasses, the effect iscaused by the incoming photons of the illuminating light exciting one ormore of the lone-pair electrons in the p-orbital of the outer shell ofthe group VI element and, having excited these electrons, anelectron-hole pair is created which no longer has the spatial symmetryof the lone-pair orbital and, due to the change of interatomic potentialleads to a swing of the chalcogen atom resulting in a local distortionof the glass leading to expansion or contraction.

Inevitably the light is absorbed by the glass and its energy istransferred to the excitation of the lone-pair electrons. Accordinglythe light is not able to penetrate far into the glassy matrix beforebeing absorbed. If reasonably strongly absorbed it typically penetratesto a depth of about 1 μm.

It is possible for the actuating member to be substantially entirelyformed by the semiconducting glassy matrix and in this case a simplechange in length of the actuating member occurs as a result of changesin its illumination. Since the change in length is only small thischange in length is difficult to detect and certainly difficult to usedirectly. Preferably therefore, the actuating member includes asubstrate which is unaffected by changes in illumination. Preferably theactuating member is formed by a layer of the semiconducting glassymatrix bonded to the substrate. Preferably the thickness of thesemiconducting glassy matrix and the substrate are comparable with oneanother. In this case when the layer of semiconducting glassy matrixexpands or contracts upon changes in its illumination, its expansion orcontraction relative to the substrate causes the actuating member tobend in much the same way as a bi-metallic strip bends in response tochanges in temperature. This results in a very much greater mechanicaldisplacement of the actuating member than that resulting from theexpansion or contraction of the semiconducting glassy matrix alone. Forexample by mounting such an actuating member as a cantilever, the freeend of the cantilever is capable of moving a distance a few orders ofmagnitude greater than the simple change in length of the semiconductingglassy matrix. Equally if the actuating member is mounted so that it isconstrained at both ends it bows and flattens with changes inillumination and its central portion again moves a distance which isconsiderably greater than that of the change in length of thesemiconducting glassy matrix.

The opto-mechanical device in accordance with this invention may be usedas a sensor to detect the presence of light or its polarisation statewith the resulting mechanical strain in the actuating member being usedto indicate the presence or polarisation state of the light. Themechanical strain of the actuating member may be monitored directly orpreferably, monitored via a secondary system. Another use of theopto-mechanical device in accordance with this invention is to create amechanical movement to do direct work, for example by using theactuating member as part of a micro- or nano-manipulator and using thechange in size of the actuating member to move a workpiece being handledby the micro- or nano-manipulator. A further application is to use theopto-mechanical device to perform a control function on a secondarysystem and thereby control the secondary system in response to changesin the light illuminating the actuating member.

The secondary system may be any one which responds to mechanical strainor movement and thus, for example, the secondary system may include anoptical lever comprising a laser beam which is reflected from theactuating member or an element connected to it and then impinges upon aposition-sensitive photo-detector so that as the actuating member movesor bends the effect of its movement turns the optical lever formed bythe laser beam so that it moves along the position-sensitivephoto-detector. The output of the position-sensitive photo-detector thengives an indication of the mechanical movement of the actuating memberand hence of the illumination applied to it. The actuating member mayinclude a metal electrode and in this case the mechanical movement ofthe actuating member and its associated electrode towards or away froman associated electrode provides a change in capacitance between the twoelectrodes. This change in capacitance may be monitored to give arepresentation of the mechanical movement of the actuating member andhence of the illumination applied to it. Equally, the substrate may beformed by a piezo-resistive material the resistance of which changes asa result of a strain imposed upon it. Accordingly, in this case, as aresult of a change in strain in the substrate caused by the strain inthe semiconducting glassy matrix, the electrical resistance varies soresulting in a change in resistance being used to provide an indicationof the change in strain of the semiconducting glassy matrix and henceits illumination. A further way in which the actuating member can beused to control a secondary system is in the case where the secondarysystem is a fluidic system. In this case the actuating member may bearranged close to a nozzle in a fluidic system so that as theillumination of the actuating member varies and hence its displacementvaries, it gets closer to or further away from the nozzle so varying therate of flow through the nozzle and hence the back pressure upstream ofthe nozzle. In this way the opto-mechanical device can be used toinfluence a fluidic system and thus produce a light sensor or acontroller which is responsive to the light input into the device which,being completely free from electrical contacts, would be intrinsicallysafe in an explosive environment.

Not only can the actuator do direct work as a micro-manipulator it isalso possible for it to do direct work as the actuating member of apump. An example of this is where the semiconducting glassy matrix isformed on a planar substrate so that with changes in its illuminationthe actuating member bows. By changing the illumination of the actuatingmember so that it alternately bows and flattens it can be used as adiaphragm of a diaphragm pump. It is also conceivable that it can beused as an optically powered motor by taking the reciprocating movementeither from such a diaphragm or from a cantilever arm and convertingthis into a rotation.

When the elements forming the semiconducting glassy matrix are innon-stoichiometric proportions the exact composition of it can betailored to influence and suit its intended use. Firstly, by selectingand varying the proportions of the element or other elements in thesemiconducting glassy matrix the band gap of the semiconductor can bevaried and so matched to the energy of the light which is used toilluminate the actuating member. Clearly this can be used to make theactuating member selective to light of a predetermined wavelength wherethe actuating member is used to detect light of a predeterminedwavelength or alternatively merely to match the actuating member to thewavelength of light output by a light source used with theopto-mechanical device and thereby optimise its sensitivity. It is alsopossible to change the rigidity of the glass material and, in this wayaffect the amount of expansion and contraction that occurs. For example,introducing iodine into the semiconducting glassy matrix appears to makeit less rigid and so causes greater expansion and contraction of thematrix for a given change in illumination. Equally, the introduction ofgermanium into the semiconducting glassy matrix appears to make it morerigid and so results in a lower expansion and contraction for the givenchange in illumination.

Another way in which the composition of the semiconducting glassy matrixcan be tailored to be given particular advantages is by changing itscomposition so that, for example, its thermal expansion is substantiallythe same as the thermal expansion of any substrate used in the actuatingmember. In this way the actuating member can be made insensitive tothermal changes so it only responds to changes in illumination. Equally,by changing the composition, the scalar effect of illumination can beenhanced or suppressed. Clearly where it is the response of theopto-mechanical device to the scalar effect of illumination which isused it is required to increase the sensitivity of the device to thescalar effect and, reduce as far as possible, the effect of any changein polarisation whereas, more usually, where it is the response to aparticular polarisation state that it is required it is important tosuppress, as far as possible, the scalar effect.

Typically the semiconducting glassy matrix may be formed by melting theingredients in bulk and then subsequently working the melt byconventional glass working techniques to provide the actuating member.Particularly where it is to be formed as a thin film on a substrate itmay be formed by sputtering, chemical vapour deposition or byevaporation. One chalcogenide glass which has shown good results whenused as the semiconducting glassy matrix is As₅₀ Se₅₀ particularly whenthis is combined with a substrate of silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

Three examples of opto-mechanical devices in accordance with thisinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic side elevation of a device including acantilevered actuating member used to demonstrate the use of thisinvention as a nano-manipulator;

FIG. 2 is graphs illustrating the change in strain of the actuatingmember upon its illumination;

FIG. 3 is a side elevation of a modified cantilever arm to a greatlyenlarged scale; and,

FIG. 4 is a diagram of an opto-mechanical device for controlling afluidic system.

DESCRIPTION PREFERRED EXAMPLES

An atomic force microscope typically includes commercially availableV-shaped microcantilevers 1. They are fabricated from silicon nitridewith typical dimensions 200 μm length, 20 μm width and 0.6 μm thickness.Such microcantilevers 1 are used in an atomic force microscope as a testrig for the present invention. One surface of the microcantilevers 1 iscoated with a thin layer, about 20 nm thick, of gold to increase itsoptical reflectivity. The other surface has a thin amorphous As₅₀Se₅₀film of thickness about 250 nm evaporated onto it. The microcantilever 1is then mounted on a cantilever holder 2 of an atomic force microscope.A helium-neon laser, the output of which is square-wave modulated ataround 50 Hz by an acousto-optic modulator (AOM) and then polarised inan electro-optic modulator (EOM) is guided via a polarisation preservingfibre 3 and used to illuminate the whole of the surface of the amorphousAs₅₀Se₅₀ film.

To monitor movement of the microcantilever 1 in this test rig, a probelaser beam 4 was focused onto the gold plated surface of the end of themicrocantilever 1 and the light reflected from the end of the cantilever1 received by a linear photodiode and optical filter 5. Any bending ordeflection of the microcantilever thus causes a deflection of the probelaser beam 4 and hence varies the output of the photodiode 5.

FIG. 2a shows a combination of two traces monitoring the movement of thecantilever when exposed to unpolarised light and polarised light in onedirection (trace A) or unpolarised light and polarised light polarisedin an orthogonal direction (trace B). This shows how, when exposed tolight of opposite polarisation states, the microcantilever bends eitherupwards or downwards. FIG. 2b illustrates the effect of changing theillumination of the microcantilever 1 from light of one polarisationstate to light of the orthogonal polarisation state. Note that thisshows that the amplitude of the bending of the microcantilever betweenopposite polarisation states is twice that between unpolarised and asingle polarisation state.

When using the microcantilever as a nano-manipulator the end of ittypically holds the sample or workpiece to be moved and then, moves thesample or workpiece as illumination of the microcantilever is varied. Itis possible to obtain a resolution of about ±1 nm in this way. Theabsence of an applied electric field to such a micro or nano-manipulatorhas considerable advantages when it is used in an electron microscope oran atomic force microscope.

FIG. 3 shows a modification of a cantilever 1 a for use in anano-manipulator. In this example the substrate 6 of the cantilever 1 ais formed as a light waveguide out of doped silica, for example. Anupper layer 7 of chalcogenide glass is then formed on its upper surface.Light used to illuminate the cantilever 1 a is then coupled into the endof the substrate 6 and this is evanescently coupled to the layer 7 ofchalcogenide glass. As an alternative the cantilever la could befabricated from an etched optical fibre with its core exposed as theactuating member.

An advantage of these modifications is that light reflected back downthe waveguide 6 has an intensity which is a function of the degree ofbending of the cantilever and so it can be used as a feedback control.

FIG. 4 shows how a device in accordance with the present invention canbe used as a pump. In FIG. 4 an actuating member 10 comprising asubstrate layer 11 and an upper layer of glassy matrix 12 forms theupper surface of a chamber 13. The chamber has a fluid inlet pipe 14 anda fluid exit pipe 15 both of which include non-return values not shownin the drawing. Light from optical fibre 16 impinges on the glassymatrix 12 of the actuating member 10 and as the polarisation of thelight leaving the optical fibre 16 changes between two orthogonalpolarisation states the actuating member 11 alternately bows into andout of the chamber 13. As the actuating member 10 flexes alternately itvaries the volume of the chamber 13 and hence acts as the diaphragm of adiaphragm pump to pump fluid from the inlet pipe 14 through the outletpipe 15.

What is claimed is:
 1. An opto-mechanical device comprising an opticallyresponsive actuating member at least partly formed from a chalcogenideglass consisting of a group VI element alone or in combination with atleast one element from the same or a different group of the periodictable, in which a mechanical strain occurs in the optically responsiveactuating member in response to changes in its illumination and in whicha change in the mechanical strain of the actuating member and hence acorresponding change in its contraction and expansion occurs in responseto changes in the state or angle of polarisation of the illuminatinglight.
 2. An opto-mechanical device according to claim 1, which includesmeans to illuminate the actuating member.
 3. An opto-mechanical deviceaccording to claim 2, which includes means to illuminate the actuatingmember with a source of polarised light and includes means to vary theangle of polarisation of the light applied to the actuating member. 4.An opto-mechanical device according to claim 3, in which the means toilluminate the actuating member is a polarisation-preserving opticalfibre and the means to vary the angle of polarisation includes anelectro-optical modulator to polarise and vary the angle of polarisationof the light applied to the actuating member.
 5. An opto-mechanicaldevice according to claim 1, in which the actuating member includes asubstrate which is unaffected by changes in illumination.
 6. Anopto-mechanical device according to claim 5, in which the thickness ofthe optically responsive component and the substrate are comparable withone another, so that when the optically responsive component expands orcontracts upon changes in its illumination, its expansion or contractionrelative to the substrate causes the actuating member to bend.
 7. Anopto-mechanical device according to claim 6, in which the actuatingmember is mounted as a cantilever, the free end of which is capable ofmoving a distance a few orders of magnitude greater than the simplechange in length of the optically responsive component.
 8. Anopto-mechanical device according to claim 6, which is arranged to act asa sensor to detect the presence of light or its polarisation state withthe resulting mechanical strain in the actuating member being used toindicate the presence or polarisation state of the light, and whichincludes a secondary system to monitor the mechanical strain of theactuating member.
 9. An opto-mechanical device according to claim 1, inwhich, in use, the actuator creates a mechanical movement to do directwork.
 10. An opto-mechanical device according to claim 1 wherein theproportions of the element or other elements in the chalcogenide glassare selected and varied to make the actuating member selective to lightof a predetermined wavelength.